Variable-Temperature and Variable-Pressure Kinetic and Equilibrium studies of formation and dissociation of [V(H2O)5NCS]2+

The kinetics of formation and dissociation of [V(H₂O)₅NCS]²⁺ have been studied, as a function of excess metal-ion concentration, temperature, and pressure, by the stopped-flow technique. The thermodynamic stability of the complex was also determined spectrophotometrically. The kinetic and equilibrium data were submitted to a combined analysis. The rate constants and activation parameters for the formation (f) and dissociation (r) of the complex are: k/M ^?1 · S^?1 = 126.4, k/s^?1 = 0.82; ΔH /kJ · mol^?1 = 49.1, ΔH/kJ · mol^?1 = 60.6; ΔS/ J·K^?1·mol^?1= ?39.8, ΔSJ·K^?1·mol^?1 = ?43.4; ΔV/cm³·mol^?1 = ?9.4, and ΔV/cm³ · mol^?1 =?17.9. The equilibrium constant for the formation of the monoisothiocynato complex is K²⁹⁸/M ^?1 = 152.9, and the enthalpy and entropy of reaction are ΔH⁰/kJ · mol^?1 = ? 11.4 and ΔS⁰/J. K^?1mol^?1 = +3.6. The reaction volume is ΔV⁰/cm³· mol^?1 = +8.5. The activation parameters for the complex-formation step are similar to those for the water exchange on [V(H₂O)₆]³⁺ obtained previously by NMR techniques. The activation volumes for the two processes are consistent with an associative interchange, I_a, mechanism.     

Variable-Temperature and Variable-Pressure 17O-NMR Study of Water Exchange of Hexaaquaaluminium(III)

The transverse relaxation rate of H₂O in Al(H₂O) has been measured as a function of temperature (255 to 417 K) and pressure (up to 220 MPa) using the ¹⁷O-NMR line-broadening technique, in the presence of Mn(II) as a relaxation agent. At high temperatures the relaxation rate is governed by chemical exchange with bulk H₂O, whereas at low temperatures quadrupolar relaxation is prevailing. Low-temperature fast-injection ¹⁷O-NMR was used to extend the accessible kinetic domain. The samples studied contained Al³⁺ (0.5 m), Mn²⁺ (0.2–0.5 m), H ⁺ (0.2–3.1 m) and ¹⁷O-enriched (20–40%) H₂O. Non-coordinating perchlorate was used as counter ion. The following H₂O exchange parameters were obtained: k = (1.29 ± 0.04) s⁻¹, ΔH* = (84.7 ± 0.3) kJ mol⁻¹, ΔS* = +(41.6 ± 0.9) J K ⁻¹ mol⁻¹, and ΔV = +(5.7 ± 0.2) cm³ mol⁻¹, indicating a dissociative interchange, I_d, mechanism. These results of H₂O exchange on Al(H₂O) are discussed together with the available complex-formation rate data and permit also the assignment of I_d mechanisms to these latter reactions.     

High-Pressure 17O-NMR. Study of Vanadium (II) in Water: A Second Example of an Associative Interchange Mechanism (Ia) for Solvent Exchange on an Octahedral Divalent Transition-Metal Ion

The water exchange of [V(H₂O)₆]Cl₂ in aqueous solution has been studied as a function of temperature and pressure (up to 250 MPa), by measuring the ¹⁷O-FT-NMR. line-widths of the free water resonance at 8.13 MHz. The kinetic parameters obtained are K = 87±4 s^?1, ΔH^* = +61.8 ± 0.7 kJ mo1^?1 and ΔS^* = ?0.4±1.9 J mol^?1 K^?1. A pressure-independent volume of activation ΔV^* = ?4.1±0.1 cm³ mol^?1 is obtained, suggesting an associative interchange (I^a) mechanism for this early divalent metal ion.     

Copper Catalysed Oxygenation of Bis (2-pyridyl) methane and 6-Methyl-bis (2-pyridyl)methane to the Corresponding Ketones

The ligands (L) bis (2-pyridyl) methane (BPM) and 6-methyl-bis (2-pyridyl)methane (MBPM) form the three complexes CuL²⁺, CuL, and Cu₂L₂H with Cu²⁺. Stability constants are log K₁ = 6.23 ± 0.06, log K₂ = 4.83 ± 0.01, and log K (Cu₂L₂H + 2H²⁺ ? 2 CuL²⁺) = ?10.99 ± 0.03 for BPM and 4.56 ± 0.02, 2.64 ± 0.02, and ?11.17 ± 0.03 for MBPM, respectively. In the presence of catalytic amounts of Cu²⁺, the ligands are oxygenated to the corresponding ketones at room temperature and neutral pH. With BPM and 2,4,6-trimethylpyridine (TMP) as the substrate and the buffer base, respectively, the kinetics of the oxygenation can be described by the rate law with k₁ = (5.9 ± 0.2) · 10^?13 mol l^?1 s^?1, k₂ = (4.0 ± 0.6) · 10^?4 mol^?1 ls^?1, k₃ = (1.1 ± 0.1) · 10^?12 mol l^?1 s^?1, and k₄ = (9 ± 2) · 10^?14 mol l^?1 s^?1.     

Zur Stabilität der Metallkomplexe von Methantriessigsäure und cis,cis-1,3,5-Cyclohexantricarbonsäure

The stabilities of the Mn²⁺-, Co²⁺-, Ni²⁺-, Cu²⁺- and Zn²⁺-complexes with 2-(carboxymethyl)glutaric acid ( 2 ) and cis,cis-1,3,5-cyclohexanetricarboxylic acid ( 3 ) were measured potentiometrically at 25° and I = 0.5 (KNO₃). Beside the complexes ML^? protonated species MLH and MLH are also formed. Their stability constants are given in Table 1. A comparison between the stabilities of 2 or 3 and those of acetate, as a model for a monocarboxylate, or succinate and glutarate, as examples for dicarboxylates, indicates that in all species only one carboxylate is strongly bound whereas the second and third ones are probably not. The observation that Δlog K₁ = log K ? log K as well as Δlog K₂ = log K ? log K are practically constants with values of 0.34 ± 0.05 and 0.49 ± 0.07, respectively, for both ligands and the five metal ions studied is also in line with the proposed monodentate structures of the complexes ML^?, MLH and MLH.     

Studies on the composition and kinetics of chromium(III)-alanine system

Kinetics of the complex formation of chromium(III) with alanine in aqueous medium has been studied at 45, 50, and 55°C, pH 3.3–4.4, and μ = 1 M (KNO₃). Under pseudo first-order conditions the observed rate constant (k_obs) was found to follow the rate equation: Values of the rate parameters (k_an, k, K_IP, and K) were calculated. Activation parameters for anation rate constants, ΔH^≠(k_an) = 25 ± 1 kJ mol^?1, ΔH^≠(k) = 91 ± 3 kJ mol^?1, and ΔS^≠(k_an) = ?244 ± 3 JK^?1 mol^?1, ΔS^≠(k) = ?30 ± 10 JK^?1 mol^?1 are indicative of an (I_a) mechanism for k_an and (I_d) mechanism for k routes (‥substrate Cr(H₂O) is involved in the k route whereas Cr(H₂O)₅OH²⁺ is involved in k′ route). Thermodynamic parameters for ion-pair formation constants are found to be ΔH°(K_IP) = 12 ± 1 kJ mol^?1, ΔH°(K) = ?13 ± 3 kJ mol^?1 and ΔS°(K_IP) = 47 ± 2 JK^?1 mol^?1, and ΔS°(K) = 20 ± 9 JK^?1 mol^?1.     

Stability,Formation and Dissociation Kinetics of the Pentacoordinate Ni2+ and CO2+ Complexes with 1,5-Diazacyclooctane-N,N′-diacetic Acid

The stability constants of the Ni²⁺ and Co²⁺ complexes with 1,5-diazacyclooctane-N,N′-diacetic acid (H₂DACODA) have been determined potentiometrically in 0.5M KNO₃ at 25°. Only M(DACODA) and M(DACODA)OH^? were observed. In addition the formation and dissociation kinetics of the pentacoordinate complexes M(DACODA) has been studied in aqueous solution using a stopped-flow technique. Formation follows the rate law v_f = k_f [M²⁺] [HDACODA^?]/[H⁺], which can be interpreted as a bimolecular process either between M²⁺ and DACODA^2? (k) or between MOH⁺ and HDACODA^? (k). The second order rate constants k are much higher than those expected from water exchange and can only be explained by a strong internal conjugate base effect. In the limiting case, however, this is equivalent to the second possible explanation, which assumes MOH⁺ and HDACODA^? as reactive species. The dissociation rate is given by v_d = (k^ML + k [H⁺]) · [M(DACODA)].     

Molecular Recognition in Anion Coordination Chemistry. Structure,Binding Constants and Receptor-Substrate Complementarity of a Series of Anion Cryptates of a Macrobicyclic Receptor Molecule

The crystal structures of four anion cryptates [X^? ? BT -6H⁺] formed by the protonated macrobicyclic receptor BT -6H⁺ with F^?, Cl^?, Br^? and N have been determined. They provide a homogeneous series of anion coordination patterns with the same ligand. The small F^?-ion is tetracoordinated, while Cl^? and Br^? are bound in an octahedron of H-bonds. The non-complementarity between these spherical anions and the ellipsoïdal cavity of BT -6H⁺ is reflected in ligand distortions. Structural complementarity is achieved for the linear triatomic substrate N, which is bound by two pyramidal arrays of three H-bonds, each interacting with a terminal N-atom of N. The formation constants of the complexes formed by BT -6H⁺ with a variety of anions (halides, N, NO, carboxylates, SO, HPO, AMP^2?, ADP^3?, ATP^4?, P₂O) have been determined. Very strong complexations are found, as well as marked electrostatic and structural effects on stability and selectivity; in particular the binding of F^?, Cl^?, Br^?, and N may be analyzed in terms of the crystal structure data. The cryptand BT -6H⁺ is a molecular receptor containing an ellipsoïdal recognition site for linear triatomic substrates of size compatible with the size of the molecular cacity. Further developments of various aspects of anion coordination chemistry are considered.     

Spectroscopic,Electrochemical, and Kinetic Characterization of New Ruthenium(II) Tris-chelates Containing Five-Membered Heterocyclic Moieties

Synthesis, redox, photophysical, and photochemical properties of Ru(NN) complexes NN = 2-((2′-pyridyl)thiazole (pyth), 2-(2′-pyrazyl)thiazole (pzth), 2,2′-bithiazole (bth), 5-(2′-pyridyl)-1,2,4-thiadiazole (pytda), 2-(2′-pyridyl)imidazole (pyim), 1-methyl-2-(2′-pyridyl)imidazole (Mepyim), and 2-(2′-pyridyl)oxazole (pyox)) are described. Oxidation potentials for the Ru^3+/2+ couples in MeCN varied from about 0.80 V to 1.60 V vs. NHE. Three reduction waves were observed in all the cases except for Ru(pyim) and Ru(Mepyim) complexes and asigned to the one-electron reduction of each bidentate ligand. Absorption spectra contained bands in the UV (280–325 nm) and VIS (437–481 nm) regions which have been assigned to ligand-centered π-π* and metal-to-ligand charge-transfer dπ-π* transitions, respectively. Emission spectra at 77 K were determined for all the complexes presenting maxima in the 580–650-nm region, with vibrational progression in some of them. Only pyth, pzth, bth, and pytda tris-chelates showed luminescence at room temperature in aqueous solution, with quantum yields ranging from 0.0013 to 0.0095 and excited-state lifetimes from 55 to 390 ns, as determined from pulsed laser techniques. Their E_0–0 spetroscopic energies have been estimated from emission wavelength maxima at 77 K which, in turn, have allowed calculation of excited-state redox potentials. A plot of E_0–0 vs. ΔE_1/2, where ΔE_1/2 = E_1/2(3+/2+) ? E_1/2(2+/+), was linear with a slope of ca. 1.1 and a correlation coefficient of 0.999, demonstrating an identical nature of the orbital involved in spectroscopic and electrochemical processes. Photochemical properties of Ru(NN) complexes have been tested using methyl viologen (MV²⁺) in Ar-purged aqueous solution at pH 5. Stern-Volmer treatment has led to the determination of bimolecular quenching constants (0.5 to 2 × 10⁹m^?1·s^?1) which parallel electron-transfer free-energy changes. Homogeneous back-reaction of primarily produced MV^+· and Ru(NN) has been measured resulting to be slightly higher than diffusion control and independent of ligand nature. Rate constants for the scavenging of Ru(NN) by added edta have been also determined (1.7 to 8.2 × 10⁸M^?1 · S^?1). Under such conditions, net production of MV^+· is attained with quantum yields varying from 0.003 to 0.038 (single-shot laser results).     

High-Pressure,High-Resolution Nuclear Magnetic Resonance (HPHR-NMR.): A Tool in Chemical Kinetics

In recent years the volume of activation Δ V* has become a powerful tool in chemical kinetics. High resolution NMR. spectroscopy is now one of the most common techniques used in the study of the kinetics of labile chemical systems. In order to measure Δ V* by this technique, we have built a ¹H-probe-head, for a Fourier transform spectrometer, working up to 4 kbar and with a resolution of 0.6 Hz. The temperature is regulated and measured with an accuracy better than 0.2°. The high pressure probe-head has been tested on a chemical system showing a dissociative-associative crossover for the ligand substitution mechanism. It had been shown previously that the ligand exchange TaBr₅ · L + *L ? TaBr₅ · *L + L proceeds via a D mechanism when L=Me₂O, and via an I_a mechanism when L=Me₂S. As expected, ΔV is positive (+30.5 ± 2.0 cm³ mol^?1) for the dissociative process and negative (?12.6 ± 1.0 cm³ mol^?1) for the associative one.     

Cuprous complexes and dioxygen. VIII. Steric factors controlling one- or two-electron reduction of O2 by cuprous complexes with substituted imidazoles

In aqueous acetonitrile (AN), Cu (I) forms the complexes Cu(AN)L⁺ and CuL with a series of substituted imidazoles (L). Stability constants logK of Cu(AN)⁺ + L ? Cu(AN)L⁺ and logβ₂ were near 5 and 12, resp., log units for all ligands. The rate of autoxidation is described by ?d[O₂]/dt=[CuL]²[O₂](k_a/(1+k_b[CuL]) + (k_c[L]+k_d)/([CuL] + k_e[Cu])), implying competition between one- or two-electron reduction of O₂. The value of k_c decreases from 5500M ^?2S ^?1 for unsubstituted imidazole to about 40M ^?2S ^?1 for 2-methylimidazole or 1,2-dimethyl-imidazole and essentially zero for the corresponding 2-ethyl-derivatives. On the other hand, k_a and k_b are much less influenced by the nature of the ligands, all values being near 5 · 10⁴M ^?2S ^?1 and 10³M ^?1, respectively, for the complexes with the last four bases. Thus rather subtle sterical changes may strongly influence the relative importance of different pathways in the reduction of dioxygen by cuprous complexes.     

Ionization Energies of 1-R-2,2-dimesitylethenols, 1,2-Dimesityl-2-phenylethenol,and Some of their Keto Tautomers

The He(Iα) PE spectra of the two pairs of 1 (Ph)- 3 and 2–4 have been recorded and assigned by comparison with spectra of related molecules, or by using simple MO arguments. This analysis shows that the PE spectra are in agreement with the assumption that the twist angles observed in the crystalline state are more or less conserved in the gas phase. The observed first adiabatic ionization energies I allow the calculation of the molar reaction enthalpy Δ_rH which accompanies the enolization of the radical cations of the title compounds. It is found that Δ_rH ≈? –?70 kJ mol^?1 is roughly the same for both keto/enol pairs, in qualitative and quantitative agreement with expectation. These results complement and correct ionization energies determined previously by mass spectrometric methods and conclusions based on them.     

Complex formation equilibria of L-Amino-Acid amides with copper(II) in aqueous solution

Protonation and Cu(II) complexation equilibria of L -phenyhilaninamide, N²-methyl-L-phenylalaninamide, N², N²-dimethyl-L-phenylalaninamide, L -valinamide, and L -prolinamide have been studied by potentiometry in aqueous solution. The formation constants of the species observed, CuL²⁺, CuL, CuLH, CuL₂H and CuL₂H_?2, are discussed in relation to the structures of the ligands. Possible structures of bisamidato complexes are proposed on the ground of VIS and CD spectra. Since Cu(II) complexes of the present ligands (pH range 6–8) perform chiral resolution of dansyl- and unmodified amino acids in HPLC (reversed phase), it is relevant for the investigation of the resolution mechanism to know which are the species potentially involved in the recognition process.     

Stability of Complex Metal Bromides in the gas phase and in toluene

The equilibrium constants of the reactions MBr₂(s) + Al₂Br₆(sln) ? MAl₂Br₈(sln) M = Cr, Mn, Co, Ni, Zn, Cd have been measured at 298 K in toluene. Ni: 0.017 ± 0.0024, Co: 0.54 ± 0.07, Zn: 1.5 ± 0.2, Mn: 2.1 ± 0, 7, Cr: 2.2 ± 1, Cd: 7 ± 5. They are compared with literature values of the equilibrium constants of analogous reactions in the gas phase MX₂(s) + Al₂X₆(g) ? MAl₂X₈(g), X = Cl, Br. For CoAl₂Br₈(sln) the temperature dependence of the equilibrium constant yielded Δ_fH = ?9.4 ± 1 kJ mol^?1 and Δ_fS = ?39.5 ± 3 J mol^?1 K^?1 while literature values for CoAl₂Br₈(g) are Δ_fH = 42.4 ± 2 kJ mol^?1 and Δ_fS = 42.9 ± 2 J mol^?1 K^?1. The solubility of Al₂Br₆ in toluene as well as its enthalpy of dissolution have been measured in order to evaluate ΔH° and ΔS° of the solvation of Al₂Br₆(g) and CoAl₂Br₈(g) in toluene by a thermodynamic cycle. Solvation of Al₂Br₆(g): ΔH = ?72.7 ± 1 kJ mol^?1, ΔS = ?139.6 ± 4 J mol^?1 K^?1, solvation of CoAl₂Br₈(g): ΔH = ?124.5 ± 4kJ mol^?1, ΔS = ?222 ± 9J mol^?1 K^?1. Thus, CoAl₂Br₈ interacts more strongly with the solvent toluene than Al₂Br₆ does.     

ESR Study of Nitroxide Radicals Generated from Triaz-2-en-1-ols

The triazenols 4-R¹? C₆H₄? N?N? N(OH)? R² ( 1 ), oxidized with t-BuO radicals, produced nitroxide radicals R¹? C₆H₄? N(O^?)? N?N(R²) ⁺O^? ( 5 ). The suggested radical structure was confirmed by ¹⁵N-labeling. The reaction of triazenols 1 with PbO₂ proceeded under N₂ elimination, in which case nitroxides R¹? C₆H₄? N(R₂)? O^?( 2 ) were observed as the final radical products. The intermediate R¹? C₆H radicals were identified by spin-trapping.     

Contribution à l'étude du système quaternaire K+?NH4+?CrO?SO?H2O: II. L'isotherme de 25°C

The study at 25°C of the system K⁺? NH? CrO? SO? H₂O has shown experimentally the existence of a new type of quaternary system of solubility with two cations and two anions. The solubility diagramm is caracterized by the presence of two adjacent ternary limiting systems with a miscibility gap, three univariant lines (one of them being evanescent), one invariant point, three binary and one ternary miscibility gaps.     

The mass spectra of organic compounds. 7th Communication. 4,4-Dimethyl-1-pentene

Loss of CH, CH₄, C₂H₄, C₃H, C₃H₆ and C₃H₇ from the molecular ions of a number of ¹³C-labeled analogs of 4,4-dimethyl-1-pentene was studied both in normal (source) 70-eV electron impact (EI) spectra dn in metastable spectra. For loss of CH in the source, 96% of the methyl comes frm positions of 5, 5′ and 5″, while the remainder comes from position 1. In the metastable spectra, loss of C-1 (16%) and C-3 (9%) is increasing in importance. The loss of ethylene is a particular case: either C-1 or C-3 are lost with any other C-atom from positions 2,5,5′, and 5″ (8 × 10%) in the metastable spectra, the probability for simultaneous loss of C-1 and C-3 being 6%. If C-1 seems to these two positions become completely equivalent in the metastable time range. The T-values (kinetic energy release) for the different positions show small, but statisticaly different values and a small isotope effect. Loss of C₃H₅ (allylic cleavage) is 100% C-1, C-2 and C-3, i.e., no evidence for skeletal rearrangement is seen. This is also true for loss of C₃C₆ (McLafferty rearrangement) within the source, but in metastable decay the other positions gain in importance. The neutral fragment C₃H appears to be the the result of consecutive loss of CH and C₃H₄, rather than a one-step loss of propyl radical or the inverse reactions sequence. No metastable reaction can be seen for this reaction. Decomposition of labeled C₆H and C₅H secondary ions occurs in an essentially random fashion.     

Some thermodynamic parameters for hydroxy amino acids: Bicine and tricine

pK values of N,N-dihydroxyethylglycine (bicine) and N-[tris(hydroxymethyl)methyl]-glycine (tricine) have been determined by the Irving-Rossotti method in an aqueous medium at 25, 30, 35, 40, 45, and 50°C and at different ionic strengths (I = 0.1, 0.5, and 1.0). Plots between pK_a(NH) and 1/T for various ionic strengths have been obtained and the values of slopes have been used to calculate the ΔH, ΔS, and ΔG for the dissociation reactions of bicine and tricine. The ΔH, ΔS, and ΔG values for bicine were found to be 10.6 ± 0.6 kcal mol^?1, ?1.9 ± 1.8 e.u., and 11.1 ± 0.06 kcal mol^?1, respectively, and for tricine 11.2 ± 0.6 kcal mol^?1, 1.6 ± 1.6 e.u., and 10.7 ± 0.06 kcal mol^?1, respectively. The pK_a(NH) values decrease with rise in temperature but the influence of ionic strength is not significant.     

Low temperature 13C NMR study of 1-(3-pentyloxyphenylcarbamoyloxy)-2-dialkyl- aminocyclohexanes

Cyclohexane and piperidine ring reversal in 1-(3-pentyloxyphenylcarbamoyloxy)-2-dialkylaminocyclohexanes was investigated by ¹³C NMR. An unusually low conformational energy ΔG = 0.59 kJ mol^?1 and activation parameters ΔG^≠₂₁₈ = 43.8 ± 0.4 kJ mol^?1, ΔH^≠ = 48.9 ± 2.5 kJ mol^?1 and ΔS^≠ = 23 ± 9 J mol^?1 K^?1 were found for the diequatorial to diaxial transition of the cyclohexane ring in the trans-pyrrolidinyl derivative. In the trans-piperidinyl derivative, ΔG₂₂₂ = 44.7 ± 0.5 KJ mol^?1, ΔH^≠ = 55.7 ± 6.3 kJ mol^?1 and ΔS^≠ = 51 ± 21 J mol^?1 K^?1 was found for the piperidine ring reversal from the non-equivalence of the α-carbons.     

Exchange Equilibrium of Oxygen Isotopes between BrO,ClO, IO and Water

The liquid phase fractionation factors α_H = (¹⁸O/¹⁶O)_H2O/(¹⁸O/¹⁶O)xo and α_D = (¹⁸O/¹⁶O)_D2O/(¹⁸O/₁₆O)xo (X = Cl, Br, I) were calculated quantum mechanically between 0 and 100°. Experimental values were obtained in the case of BrO at 60° showing good agreement with the calculated results.